A meltdown is the out-of-control melting of a superheated reactor core, which would become so hot that nothing could conitain it. Experts differ on what would happen next. One early theory was that the core would fall through the earth and just keep going, "all the way to China" This scenario was later pushed aside, though not before it spawned a nuclear disaster movie starring Jane Fonda called We China Syndrome (1979). Now some believe that it would simply wind up encased in volcanic glass about one hundred feet below the ground, sealed safely. Others disagree, suggesting that the breach of the containment dome would release fantastic amounts of radioactivity into the surrounding area, killing thousands, perhaps hundreds of thousands. One thing is clear: Unit 2 of the Three Mile Island nuclear power plant came perilously closerto a full-scale disaster. The Presi-dential Commission report suggests that if...